The present invention relates to a thermally actuated switch, and particularly, one in which a ferrite material is employed to effect switch operation at its Curie point temperature.
Thermally operated, encapsulated reed switches which employ ferrite for switching at the Curie point temperature are well-known. U.S. Pat. No. 3,295,081 represents an early design of such a switch and particularly one which switches from a normally closed position to an open position as the temperature increases. U.S. Pat. No. 4,325,042 discloses a variety of prior art switches of this general category and an improvement thereon in which the switch switches between an open and a closed position and back to an open state at two different temperatures. Switches which generally are open and switch to a closed position as temperature increases will be referred to hereinafter as an A-type switch. Switches which switch between a closed to an open position with an increase through the Curie point temperature are B-type switches.
It is well-known to those skilled in the art that A and B switch designs differ in the placement of toroidal magnets surrounding a sealed reed switch together with ferrite material placed typically adjacent the magnets. The mechanism by which the switch will change from an open to a closed state involve the attractive forces generated by the magnetic fields applied to the reed contacts at the tip ends of the contacts and over the length of the switch arms by the permanent magnets acting upon the reed switch in conjunction with the ferrite poles associated with the magnets.
Typically, it has been discovered that with A-type switches, in particular, there exists a relatively wide and variable reset temperature range, i.e., the temperatures at which the switch changes from an open to a closed state and then back to an open state, are excessive for some applications. While the B-type switches may have a temperature reset range of 1.degree. C., typical A-type reset temperatures can be 21/2.degree. C. or more. Also, switching temperatures from unit to unit can vary quite widely in prior art A-type switches. These problems are believed to be due to the fact that the ferrite material is not perfectly homogeneous material due in part to zinc evaporation and relative zinc depletion at the surface. Further, transition of the material from a high permeability state at below the Curie temperature to a low permeability state above the Curie temperature is not sharply defined but includes a para-magnetic transition zone following the known second order Curie-Weiss relationship.
The effect of the relatively wide and variable reset temperature ranges and switching temperatures is that switches which will meet design criteria are relatively expensive due to a relatively low yield rate during quality control testing.